1. Field of the Invention
This invention relates to methods and apparatus for welding by use of high frequency electrical resistance heating and, more particularly, to the welding of brake webs to brake tables to form brake shoes.
2. Description of the Prior Art
Traditionally, brake shoes have been formed by a series of projection welds between the brake table and the brake web. This has been accomplished by providing projections on a flat plate or table to concentrate the power flowing through the web. The web and the table are rolled together and resistance welded at the projections. The projections provide the metal that forms the weld to hold the brake shoe together. In the present invention, there are no projections. In the present invention, each brake web has a die brake which can be defined as a small angular projection of metal which has been left by the die in the machine which stamps out the curved web pieces from flat metal plate. During welding the die brake melts and is forced into contact with heated metal of the brake table to provide most of the metal for the weld.
If one were to attempt to utilize the old method of resistance welding to form a continuous weld as is formed in the brake shoe of the present invention, two deleterious conditions would occur. First, the tremendous amount of heat required would melt so much of the web metal that the outer radius of the web would lose its shape thereby distorting the final brake shoe. In addition, the heat required would melt so much of the metal in the brake table that the thickness of the metal in the table under the web table interface would be greatly reduced. The combination of these two effects would result in a brake shoe that had a weaker web table weld interface than a brake shoe made by a series of projection welds of the same type.
By using a high voltage radio frequency current, a continuous weld of fused metal from the table and the web can be produced without any of the deleterious effects that occur in the prior art. Since the use of high frequency (200,000 Hz or more) versus the low frequency (60 Hz) prior art resistance welding brings about current densities in the order of 1 million watts per cubic inch at the weld point between the web and the table heating can be localized. When the localized heating is coupled with a rapid controlled advance of the weld point, the amount of fusion between the web and the table and the resultant melting thereof can be very closely controlled. The weld of the present invention uses only the die brake on the web, which is about 0.017 inches, as the metal utilized for fusion. Therefore, the dimensions of both the table and the web remain relatively unchanged.
It is not desirable to use arc welding in making brake shoes since this requires fillet welds on either side of the web-table interface. These fillets (usually 3/16") may impinge on the rivet holes which are placed relatively close to the web to enable the braking material to be riveted on the brake shoe. Also, in a brake shoe with two webs, as is the case in the preferred embodiments discussed below, it is difficult to position two weld electrodes within the space between the two webs. In addition, the extra cost of slow weld speeds, weld wire and shield gas must be considered.
As will be better described below, the welds produced by the present invention are of superior strength when compared to the prior art resistance welding methods.
There are many examples of prior art devices which utilize radio frequency welding to form various continuous welds of strips and the like. U.S. Pat. No. 2,821,619, issued Jan. 28, 1958 to W. C. Rudd discloses the basic method of using high frequency electrical resistance to weld a continuous strip for a metal flange.
U.S. Pat. No. 3,513,284, issued May 19, 1970 to J. N. Snyder discloses an apparatus which uses high frequency resistance heating for welding an edge of a web member to the face of a flange member to form a long structural shape. This apparatus cannot be used for the welding of short sections as is the apparatus of the present invention.
U.S. Pat. No. 3,375,344, issued Mar. 26, 1968 to F. Kohler et al discloses a method and apparatus for simultaneously welding elongated metal members together at two spaced weld points using high frequency electrical current. Again, the apparatus disclosed is used to weld structural shapes out of long strips of metal and not short pieces as in the present invention.
U.S. Pat. No. 3,391,267, issued July 2, 1968 to W. C. Rudd also shows an apparatus for welding long strips to form structural shapes such as I beams. This patent also discloses a method of welding finite length flange sections to the web as long as the flange sections are in end to end contact. If this were not the case, as the patent points out, there would be weld interruptions or irregularities in the weld seam and a foot or more of the welded beam structure would have to be cut off and wasted where the trailing and leading ends of the successive strip pieces pass through the welding zone. In the present invention, the entire weld length may be no more than 14 inches and the method described below must be used to insure that a high quality weld is formed almost to the end of the brake table brake web interface.